This application is based on an application No. 239725/2000 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a color toner for developing an electrostatic image.
2. Description of the Related Art
In an electrophotograph process, it is a general manner to form a fixed image by way of plural processes wherein (i) a latent image is electrostatically formed on a photosensitive member comprising a photoconductive material by various means, (ii) the latent image is developed by using a toner, (iii) the toner image on the photoconductive member is transferred to a member to be transferred (e.g. paper and the like) by way of an intermediate transfer member or by a direct manner, and then (iv) the transferred image is fixed on the member to be transferred. Recently, the electrophotographic process has been extensively utilized in not only a copying machine but also a printer since the machinery and tools have been developed and the communication networks are expanded in an information-oriented society, and therefore miniaturization, lightening, speeding up and high dependability of the apparatuses have strictly been required.
In case of a color electrophotography, high image quality and high color development are required of the formed image. In order to obtain the image having high image quality and high color development, the toner must sufficiently be melted to form the fixed image having a smooth surface from viewpoints of light transmission (particularly light transmission for OHP), glossiness and the like. For this reason, a fixing process is especially important in the electrophotographic process.
As a contact-type fixing method which has formerly been used widely as a fixing method, a method wherein heat and pressure are applied at the time of fixing (hereinafter referred to as heat-contact bonding method) has generally been employed. In the case of the heat-contact bonding method, it is known that a heat efficiency is extremely excellent and the fixing can quickly be carried out, and particularly said method is very effective for a high-speed electrophotographic copying machine, because a surface of a fixing member is contacted with the toner on the member to be transferred under pressure. However, the aforementioned heat-contact bonding method causes a problematic phenomenon that a part of the toner image is adhered to the surface of the fixing member and is transferred to nonimage part of the member to be transferred since the toner image is contacted in a heated and melted state with the surface of the fixing member under pressure (a high temperature offset). In particular, it is necessary for a fixing of the color toner to apply a sufficient heat and pressure to the color toner to afford it a higher flowability in comparison with a fixing a black-and-white toner since it is necessary in the former to melt and mix the plural colored toners. Therefore it has been more difficult to release the toner image from the fixing member in the former in comparison with a releasability of the latter because it is necessary to release the melted thicker layer formed by superposing the plural colored toners without causing the high temperature offset.
Accordingly, a method wherein silicone oil and the like are coated on the surface of the fixing member as a liquid for preventing the high temperature offset has been employed as a convenient method for preventing the adhesion of the toners on the surface of the fixing member. However, and oil adhesion after the fixing to the member to be transferred and the image has been come into question since it is necessary to use a relatively large amount of the oil in order to prevent the high temperature offset by the oil only at the time of forming the color image. Furthermore, in the case where the oil is employed, there are problems that (i) a fixing apparatus requires a tank for storing the oil and the like, (ii) the tank makes the miniaturization of the apparatus difficult, (iii) a supplement of the oil is troublesome, and (iv) a low-costing is restricted.
In U.S. Pat. No. 4,968,574, a black-and-white toner is disclosed which exhibits an excellent oilless fixing aptitude which is caused by a combination effect of a wax and a resin having a wide molecular weight distribution, said resin comprising a low molecular weight component and a high molecular weight component. According to this technique, the toner image on the member to be transferred is remained as it is in order to prevent the high temperature offset by a rubber elasticity which is attributable to the high molecular weight component. However, when this technique is applied to the fixing of the color image, there is a problem that a glossiness of the fixed image obtained becomes lower, and a color development of the color image is reduced. In addition, the aforementioned problem of the high temperature offset cannot be solved completely by this technique since there are instances where the color image is formed by a superposition of three or four toner layers.
The Japanese Patent Publication (KOKAI) Nos. 174061/1988, 174062/1988 and 174063/1988 disclose the techniques wherein a resin prepared by using isocyanate, a resin having atom group of xe2x80x94NHCOxe2x80x94 in the molecule, and a low molecular crystalline resin having a strong cohesive energy (amide wax) are employed respectively. According to these techniques, although the fixing at low temperature can be achieved, the offset resistance is insufficient. Furthermore, although the effect of preventing the offset can be brought about by a permeation of the molted toner into paper, a uniform image having high density cannot be obtained.
The present invention has been made in view of the aforementioned situation. The object of the present invention is to provide a color toner for developing an electrostatic image having excellent fixing efficiency, said toner effectively preventing the low temperature offset and the high temperature offset when it is applied to the fixing device whose coating weight of an oil is reduced. Another object of the present invention is to provide a color toner for developing an electrostatic image having excellent fixing efficiency, said toner having a relatively wide and low temperature range which does not bring about the low temperature offset and the high temperature offset.
The present invention relates to a color toner characterized in that (i) it comprises a binding resin and a colorant, (ii) its storage elastic modulus at 90xc2x0 C. (Gxe2x80x290) is not more than 6xc3x97104 Pa, (iii) its storage elastic modulus at 140xc2x0 C. (Gxe2x80x2140) is not less than 5xc3x97102 Pa, (iv) a temperature showing a maximum value of xcex4 in tanxcex4=Gxe2x80x3/Gxe2x80x2 (wherein Gxe2x80x2 is a storage elastic modulus of the color toner, and Gxe2x80x3 is a loss elastic modulus of the color toner) exists in the range of 90-120xc2x0 C., and (v) the maximum value of xcex4 is not less than 60xc2x0.
The present inventors have paid our attention to a behavior of the toner at the specified temperature range, and have found that the aforesaid objects can be achieved by increasing an elasticity of the toner at the time of melting (fixing) the toner and by decreasing the elasticity of the toner at a relatively low temperature.